1. Field of the Invention
The present invention relates to a honeycomb structured body.
2. Discussion of the Background
It has been a problem in recent years that various substances contained in exhaust gases that are discharged from internal combustion engines of automobiles and the like are harmful to the environment and the human body, and regulations against exhaust gases have been strengthened.
For example, exhaust gases discharged from gasoline engines used in many private cars and the like normally contain HC, CO, NOx and the like. In order to remove these substances, there has been developed an exhaust gas conversion system in which, in an exhaust gas passage, a catalyst supporting member with a three way catalyst supported thereon is installed.
FIG. 6 is a cross-sectional perspective view that schematically shows a catalyst supporting member used in such an exhaust gas conversion system.
As shown in FIG. 6, a catalyst supporting member 70 is a honeycomb structured body made of a porous ceramic in which a large number of through holes 71 are longitudinally placed in parallel with one another with a wall portion 73 therebetween. The three way catalyst is supported on the wall portion 73.
By allowing exhaust gases to flow through these through holes 71 to be made in contact with the three way catalyst, HC and CO in the exhaust gases are oxidized, while NOx is reduced; thus, the exhaust gases are converted.
At this time, only in a narrow range in which the air-fuel ratio of a mixed gas injected into the gasoline engine is kept in the vicinity of the theoretical air-fuel ratio, the three way catalyst exerts a high converting ratio for HC, CO, NOx and the like in exhaust gases; therefore, the air-fuel ratio of the mixed gas injected into the gasoline engine is controlled to be set to the theoretical air-fuel ratio.
However, since a diesel engine used in many large-size transporting means such as buses and trucks burns a fuel in a lean-burn state (excessive oxygen state) at a higher temperature in comparison with the gasoline engine, the content of NOx is comparatively high although the contents of HC and CO in the exhaust gases are smaller in comparison with exhaust gases discharged from gasoline engines, and SOx derived from sulfur contained in light oil serving as the fuel is also contained. Moreover, many particulate substances (particulates) are contained therein to cause a major problem.
FIG. 7 is a partially notched perspective view that schematically shows a honeycomb structured body (honeycomb filter) used for collecting particulates contained in exhaust gases discharged from diesel engines.
As shown in FIG. 7, a honeycomb filter 80 has a honeycomb structure made of a porous ceramic in which a large number of through holes 81 are longitudinally placed in parallel with one another with a wall portion 83 therebetween. Either one of the ends at the inlet side or outlet side of the through hole 81 is sealed with a plug 84 to form a so-called diced pattern; thus, exhaust gases, entered through one through hole 81, are allowed to flow out of another through hole 81 after always passing through the wall portion 83 that separates the through holes 81. When the exhaust gases pass through the wall portion 83, particulates are captured at the wall portion 83, and thus the exhaust gases are purified.
In the honeycomb filter of this type, for elimination by oxidization or reduction of HC, CO, NOx and the like contained in the exhaust gases and also to collect particulates in the exhaust gases, a catalyst for removing these substances is supported.
Moreover, when particulates exceeding a predetermined amount have been collected on the wall portion of the honeycomb filter, the pressure loss becomes so high that the filter is no longer usable; therefore, it is necessary to carry out a regenerating process to regenerate the honeycomb filter by thermally decomposing and removing the collected particulates.
In recent years, in order to carry out the regeneration process of such a honeycomb filter at low energy with high efficiency, there has been used a method in which a catalyst for oxidization-removing particulates is supported on the wall portions of the honeycomb filter.
Normally, the burning temperature of the particulates (soot) is in the range of about 550° C. to about 630° C.; however, by supporting the catalyst for oxidization-removing the particulates on the wall portions of the honeycomb filter, the burning temperature of the particulates can be lowered through its catalytic function.
With respect to the honeycomb filter in which the catalyst of this type is supported, there is a well known filter in which, for example, a supporting layer made of γ-alumina is formed on the surface of each of filtering walls (cell walls) of a heat-resistant supporting member formed by molding cordierite or the like into a honeycomb filter as shown in FIG. 7, and on the supporting layer, a catalyst active component including noble metal such as Pt, Pd, Rh or the like is further supported (hereinafter, also referred to as supporting method (1)).
With respect to the supporting layer, a layer has been proposed in which: fine powder, obtained by adding an inorganic binder to γ-alumina and pulverizing the mixture, is formed into a slurry, and a so-called wash-coating process in which this slurry is uniformly sprayed on the wall face of a honeycomb supporting member made of cordierite, is carried out so that an alumina layer is formed (for example, see JP-A 05-68892).
As indicated by JP-A 2001-137714, the present inventors have developed a technique in which each of particles forming a porous ceramic supporting member has its surface coated with an alumina thin film, with a catalytic active component being supported on the irregular surface of the alumina thin film (hereinafter, also referred to as supporting method (2)).
The contents of JP-A 05-68892 and JP-A 2001-137714 are incorporated herein by reference in their entirety.